It is known that relaxin is a polypeptide hormone produced by the ovaries, uterus and possibly the placenta in several species. The physiology of relaxin remains poorly understood in the normal female reproductive cycle and in the male. The physiology of relaxin is currently being investigated in the pig, sheep and human using a radioimmunoassay developed to the NIH-R-P1 porcine ovarian relaxin and a radioimmunoassay for highly purified porcine ovarian relaxin isolated from pregnant sow ovaries. Radioimmunoassays will be developed for relaxins isolated from human and porcine uteri and human placentae. Physico-chemical comparisons will be made between the various relaxins isolated and the relationships between the precursor and hormone, hormone and metabolites will be studied. Radioreceptor assays which have been developed will be further characterized and these techniques used to identify relaxin target tissues other than the pubic symphysis. BIBLIOGRAPHIC REFERENCES: Chamley, W.A., Stelmasiak, T. and Bryant, G.D. (1975) Plasma relaxin immunoactivity of the sheep oestrous cycle. J. Repr. Fert. 45, 455-461. Bryant, G.D., Panter, M.E.A. and Stelmasiak, T. (1975) Immunoreactive relaxin in human serum during the menstrual cycle. J. Clin. Endo. Metab. 41, 1065-1069.